1. Field of the Invention
The present invention relates to an elementary coaxial cable wire, and a coaxial cable and a coaxial cable bundle using the above elementary coaxial cable wire.
2. Description of Related Art
It is known that wire cables used for signal transmission in medical equipment such as a diagnostic probe in a ultrasonic diagnostic apparatus, and an endoscope, as well as in an industrial robot; and wire cables used for internal connection in information equipment such as a notebook-sized personal computer are repeatedly bent during use. This causes strain to be accumulated in the wire cable and there is a possibility that a break in the wire cable may occur.
Accordingly, in order to increase bending resistance of the wire cable, a stranded wire formed by twisting a number of thin wires together is broadly employed as the core conductor of a coaxial cable (or an elementary coaxial cable wire thereof), i.e., as the wire cable. One example of such a coaxial cable is shown in Japanese Laid-Open Patent Publication No. 9-35541, in which a wire made of conductive fiber reinforced copper matrix complex and a cable formed by this wire are disclosed.
The inventors in the present application have applied themselves closely to the study of the above prior art wire and cable using such a wire. As a result, they have found that the prior art involves the following drawbacks, that is:
(1) Although the prior coaxial cable using a stranded wire as a core conductor thereof has good flexibility, the configuration of the stranded wire formed by twisting thin wires together may collapse or loosen when crimped or pressed. Thus, a break in the core conductor may occur when the coaxial cable is in service;
(2) When the end of the core conductor is soldered to a circuit board having a pattern of fine-pitch traces, short circuits may be possibly caused, as the thin wires come undone due to the loosening of the stranded wire. Thus, the connection and successive processing of the coaxial cable end, including inspection of manufactured coaxial cables, become very complicated; and
(3) Furthermore, the stranded wire not only involves the above disadvantages, but also requires much expense in time and effort to manufacture the same. Thus, it has been desired to provide a coaxial cable not only having good flexibility, but also capable of decreasing manufacturing and connecting costs.
The present invention has been made in view of the above circumstances and has for its object to provide an elementary coaxial cable wire, a coaxial cable, and a coaxial cable bundle, each of which has a sufficient bending resistance, can effectively prevent a break or a short circuit in the connection, and can achieve greater economy.
The inventors have repeated diligent studies and, as a result, have found that there is a close correlation between a tensile strength and material of a core conductor, and a bending resistance of a wire. The present invention has been invented based on the above finding.
According to one aspect of the invention, an elementary coaxial cable wire comprising a core conductor, an insulator surrounding the core conductor, and an outer conductor surrounding the insulator is provided. The elementary coaxial cable wire is characterized in that the core conductor is made of a metallic material including copper and silver so as to have a tensile strength of 120 kgf/mm2 (kg/mm2) or more and an electrical conductivity of 60 to 90% by IACS (International Annealed Copper Standard).
Although a high bending resistance is required to elementary coaxial cable wires and coaxial cables used for the above-described applications, the prior art solid single wire consisting of the copper-containing metallic material could not exhibit the required bending resistance, resulting in a relatively short bending life (the number of bending times until fracture may be small). However, it has been determined that the elementary coaxial cable wire being constructed as above-explained according to the present invention has a very extended life despite the solid single wire employed as the core conductor. Generally, the fatigue limit increases with the tensile strength and this is substantially applicable to the bending characteristics. The more the tensile strength increases, the more the bending characteristics become superior.
Preferably, the core conductor has a plastic elongation of L in %, which meets the requirements expressed by the following equation (1):
0.2%xe2x89xa6Lxe2x89xa62.0%xe2x80x83xe2x80x83(1)
Bending tests were conducted on coaxial cables using conductors as a core conductor having the same tensile strength of 120 kgf/mm2 or more but different in plastic elongation. As a result, it has been shown that the coaxial cables using the core conductors, of which plastic elongation falls within the range expressed in the equation (1), have bending life longer than that of the coaxial cables using the core conductors, of which plastic elongation is below the lower limit in that range. Thus, in point of the bending characteristics, the coaxial cables according to the present invention are far superior to the prior art cable.
This tendency is more notable in the coaxial cable according to the present invention, in which the core conductor comprises the solid single wire, as compared to the prior coaxial cable including the core conductor composed of the stranded wire. It is considered that in the bending tests, a strain larger than plastic elongation is created in the surface of a core conductor and with the specific configuration of a coaxial cable, plastic elongation of the core conductor according to the present invention becomes larger than the prior art. Therefore, it is expected that under the above conditions, the occurrence of a crack and the propagation thereof in a surface portion of the core conductor which is subject to the largest strain is more restricted in the core conductor forming the elementary coaxial cable wire according to the present invention.
It is also noted that as the core conductor is composed of a solid single wire, the configuration of the core conductor will not be easily collapsed when it is forcedly pressed at connectorizing process. Thus, a break in the core conductor is prevented from occurring when the elementary coaxial cable wire is in service. Furthermore, when the core conductor is soldered to a circuit board having a pattern of fine-pitch traces, the core conductor does not become loose, so that occurrence of short circuits can be effectively prevented. Thus, not only the load of inspection during the connection is significantly lightened, but also the number of man-hours needed to perform the connection and successive processing for the elementary coaxial cable wire can be surprisingly diminished.
Furthermore, as the core conductor is a solid single wire, the core conductor can be deformed so as to have a uniform cross section when the terminal end of the elementary coaxial cable wire is swaged by a press and so on, provided that pressure conditions and so on are maintained constant. Thus, it is possible to surely connect a plurality of elementary coaxial cable wires, as a single unit, to the corresponding connecting points on such as a substrate. In other words, the elementary coaxial cable wires can be easily connected, so to speak, in a manner similar to that in which an integrated circuit (IC) is deposited on such a substrate as a circuit board. As a result, the number of man-hours needed to perform the connection and successive processing of the elementary coaxial cable wires can be further surprisingly diminished.
As the conductivity is within the above range, it is possible to prevent increased transmission loss due to Joule heat created within the core conductor. Thus, as the increased loss of transmission due to Joule heat created within the core conductor during the signal transmission can be prevented, it is not necessary to increase the core-conductor diameter in order to restrict the loss of transmission.
Furthermore, conductivity and tensile strength are mutually contradictory. However, if the metallic material contains predetermined amounts of copper and silver, it is possible to provide the core conductor with the high conductivity and the high tensile strength, both being within the above ranges. The insulator may be preferably made of a flexible material so as to decrease the possibility that the insulator may break during the bending of the elementary coaxial cable wire.
Preferably, the metallic material has a silver percentage content of 2 to 10% in weight. By using the material having the preferable composition as above described, it becomes possible to enable the core conductor comprising the solid single wire to have surely the above values of tensile strength and conductivity. The core conductor may be manufactured in a manner similar to that conventionally used in forming of wires. It is thus possible to ensure that the bending characteristics of the elementary coaxial cable wire are surprisingly improved. In addition, the core conductor is not easily collapsed even if it is forcedly pressed and short circuits can be prevented. Thus, the number of man-hours needed to perform the connection and successive processing can be surprisingly lessened.
Further, the inventors in the present invention have applied themselves closely to the study of the mechanism of break in the core and outer conductors and found the conditions under which favorable bending characteristics can be obtained. That is, tensile strengths Tc and Tg of the core and outer conductors respectively preferably meet the requirements expressed by the following equation (2):
Tgxe2x89xa6Tcxe2x89xa6Tgxc3x973xe2x80x83xe2x80x83(2)
If a value of Tc falls within the above range, it is possible to prevent the stress from being concentrated in the bend of either the core conductor or the outer conductor during the bending motion of the elementary coaxial cable wire. This means that a plastic deformation possibly occurring in one of the core and outer conductors does not increase over that occurring in the other. As a result, it is possible to prevent bending resistance of one of the core and outer conductors from being excessively decreased relative to that of the other.
Preferably, the core conductor has a diameter of 0.010 to 0.2 mm, more preferably 0.020 to 0.15 mm. Generally, bending tests are performed on mandrels (metallic bars or rods) having the same diameter, with the same load being applied thereon (refer to Methods of Flexural Testing, which will be explained below). In the bending tests, if the diameter is below 0.010 mm, bending life of the core conductor will be tend to decrease remarkably due to a stress applied on the core conductor. On the other hand, if the diameter exceeds 0.2 mm, a strain applied on the core conductor will be so large that the bending life is also reduced.
According to another aspect of the invention, a coaxial cable preferably comprises the aforementioned elementary coaxial cable wire, and a sheath surrounding the elementary coaxial cable wire. As stated above, the elementary coaxial cable wire according to the invention has the surprisingly increased bending resistance. If the sheath is flexible, the coaxial cable also has a sufficiently increased bending resistance. Furthermore, as the elementary coaxial cable wire can be very easily connected to connecting points on such as a circuit board, or a connector, the number of man-hours needed to perform the connection and successive processing can be surprisingly diminished.
Preferably, the coaxial cable according to the present invention comprises a plurality of elementary coaxial cable wires arranged in a row within the sheath. This enables the coaxial cable to have an increased bending resistance, especially when bent around an axis along the row of the elementary coaxial cable wires. The coaxial cable may be formed thinner than that possible in such an arrangement in which elementary coaxial cable wires are not disposed in a row. Thus, the coaxial cable may be laid in a narrow space within a device and so forth.
According to further aspect of the invention, a coaxial cable bundle is provided, which includes a plurality of coaxial cables according to the present invention, the coaxial cables being disposed within a sheath. With the multi-coaxial cable having the above configuration, because the sheath has the good flexibility, the bending resistance inherent in each coaxial cable can be maintained. This enables the coaxial cable bundle to have a bending resistance at least equal to or greater than the bending resistance of the coaxial cables.
Furthermore, as the terminal end of each elementary coaxial cable wire may be processed in a uniform configuration, the coaxial cable bundle can be surely and easily connected to connecting points on such as a circuit board, or a connector. In consequence, the number of man-hours needed to perform the connection and successive processing of the coaxial cable bundle can be lessened.
In the present invention, the terms xe2x80x9ctensile strengthxe2x80x9d and xe2x80x9cplastic elongationxe2x80x9d are defined in JIS C 3002 and a value of xe2x80x9celectrical conductivityxe2x80x9d are determined in accordance with JIS C 3001.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.